Pumps for delivering drugs to a human or animal subject range from simple cheap devices to expensive, complicated microchip-controlled devices. Amongst the cheapest and least complicated devices are osmotic and elastomeric pumps.
Osmotic pumps such as the "ALZET" (Trade Mark) pump produced by the Alza Corporation, Calif., rely on osmotic pressure developed between an aqueous environment and an osmotic solution to drive a drug from a reservoir. Elastomeric pumps such as the "INFUSOR" (Trade Mark) pump produced by Baxter Healthcare Corporation, Ill., rely on the contraction of an expanded elastomeric reservoir to drive the drug from the reservoir. Both types of pump are advantageous primarily as a result of their simplicity, cheapness and reliability. Developments in technology have led to a situation where a highly reliable rate of drug delivery can be obtained from such devices.
Similar simple devices include pumps where the contraction of the reservoir is driven by a spring, by the generation of gas in a simple electrolytic cell, or the generation of gas by a chemical reaction. Again, cheap devices having a reliable delivery rate are available using such technologies, although even in devices employing an electrolytic cell the expense and complexity is increased by the necessity of including a battery.
The primary limitation with such devices, some of which have been available since the 1970s, is that they are only suitable in situations where a steady, continuous flow of drug is desired. This immediately precludes their use where a variation in the drug flow rate is required. One particular example of such a case is if a pulsatile drug flow is desired. This method of delivering drugs has become important in recent years. A good overview of the types of situations where a pulsatile drug flow is advantageous can be found in "Pulsatile Drug Delivery--current applications and future trends" ed. Gurny, Junginger and Peppas (published by Wissenschaftliche Verlagsgesellschaft mbH, Stuttgart, 1993). One of the main reasons why pulsatile drug delivery may be advantageous is that a constant supply of drug does not always give rise to constant effects. Furthermore, the body may respond better, in the case of some drugs, to a pulsatile delivery regime.
While other devices have certainly been disclosed which can be used to effect pulsatile drug delivery, such devices in no way approach the simplicity or cheapness of the basic single rate devices referred to above. For example, the devices may rely on microprocessors or electronic timing circuits to switch delivery on and off, or they may employ continuous gas generation which is relieved periodically by a valve which snaps open to effect a periodic cyclical generation of driving pressure. These devices employ mechanisms which are in many cases less reliable than single rate devices, and they are generally significantly more expensive to produce.
The present invention seeks to overcome these disadvantages and to provide pulsatile delivery devices which are simple, cheap and can employ the existing technology which has proved successful in relation to single rate drug pumps.